The technique of material processing through the utilization of plasmas has been known for many years. In one application, it may involve the production of thin films on a surface through the action of the plasma, while in another it may involve the etching of those same films, also through the action of the plasma. In recent years the commercial demand for the equipment involved has increased significantly as its application to the creation of microchips and other semiconducting devices has been refined. Basically the technique involves the ignition and maintenance of a processing plasma through the application of electric power to the plasma. The plasma then interacts with gases introduced and the surfaces involved to effect the processing desired.
A potentially important characteristic of the field involved is that this field has evolved with a primary focus upon the plasma itself, not the circuitry involved. Although those skilled in the art have long desired certain refinements in electrical circuitry and capabilities, the suppliers of AC power generation equipment and handling equipment for plasma processing have generally applied techniques long known in the communication field, without always recognizing the differences between the nonlinear plasma load and the linear antenna (the usual load for communications equipment). While plasma-oriented physicists and chemists have greatly expanded the understanding of the processes involved, advancements with respect to the alternating power supplied and the circuits involved have not occurred to the same degree.
As those skilled in the art have come to understand, the nature of the processing plasmas being utilized does not easily lend itself to modeling as a simple circuit element in an AC circuit. Rather, with even detailed circuit knowledge, the inclusion of a plasma in an AC circuit is inherently difficult. This is because in most processing environments the plasma acts not just as an active element--that is, one whose characteristics depend upon other system parameters such as gas pressure and temperature--out it also acts as a highly nonlinear and dynamic element. As a nonlinear element, the plasma changes its electrical characteristics depending upon the power applied to it in a discrete fashion throughout the range of operations and environments. Naturally, when the plasma is quenched, the chamber may have high impedance; when ignited, it may have low impedance. However, the class also varies highly even while it is ignited. This effect is particularly acute in instances where alternating power is utilized because vast variations in the character of the plasma can occur even over one cycle of the alternating power, voltage, or current. The plasma's nonlinear character has made traditional circuit analyses extremely difficult and has made understanding instabilities and oscillations more difficult. As a dynamic element, the plasma can change its character over even one cycle of power. Again, this makes understanding problems encountered more difficult. These difficulties are compounded when switch-mode power supplies are used. Wile alternating power generators have been used for many years to excite a plasma, the desire to utilize solid state, switch-mode generators for reliability and smaller size has resulted in certain new problems.
The highly variable nature of the plasma can often lead to characteristics which are not conducive to stable, continuous operation. This problem is particularly acute in situations where switch-mode power supplies are utilized. In such instances, it is not uncommon for the supply of alternating power to be suddenly and inexplicably interrupted or to be altered in some fashion. These conditions may exist until operator intervention or in some instances they may also suddenly and inexplicably cease to exist on their own. Until the present invention, the exact cause of these conditions was not understood. Rather, those skilled in the art merely assure that it was incidental by involvement with the plasma due to its highly variable nature. Those skilled in the art were however able to identify that the effects seemed to manifest themselves in two different ways. First, oscillation of the switch-mode power supply might occur. Second, the waveform of the power supply might be affected such that efficiency would drop. This could even occur to the extent that a different class of operation might result. Perhaps as a result of the latter of these two manifestations, the response by those skilled in the art was to overcome the problem by avoiding switch-mode power supplies altogether. By utilizing power supplies operating in a nonswitch-mode class (typically Class B operation), it was understood that these effects would largely be avoided. This was especially true for relatively inefficient power amplifiers because impedance changes in the plasma would thus represent a relatively lower percentage change to the entire system.
Prior to the present invention, however, no solution which reliably overcame these instability effects existed for systems utilizing switch-mode power supplies. Even though the art of stabilizing power supplies in general is well known to encompasses theories originally advanced by Linvill and Stern, these approaches did not appropriately apply to plasma systems using switch-mode amplifiers. These approaches are premised upon linear elements and are generally applied to broadband amplifiers. The application of the present invention involves not only nonlinear elements, but it also presents a narrowband system, thus efforts to solve the problems encountered using traditional stability theories have not been successful. While those skilled in the art had long expressed a need to overcome these problems for switch-mode power supply applications, the solutions implemented actually taught away from the desired direction in that they avoided the use of the relatively high efficient, switch-mode power supplies in the first place. As the present invention discloses, efforts by those skilled in the art were inadequate because they failed to understand the problem thoroughly enough to implement a solution. This is poignantly shown by the relatively simple nature of the present invention and the unexpectedly simple implementation which has proven to overcome the problems. Until the present invention, however, these similar solutions were not available to those skilled in the art.